Linux Kernel 2.6.37 启动过程：startup_32

http://blog.chinaunix.net/uid-1701789-id-148056.html

一路走来真是内牛满面，现在终于来到了kernel的32位入口了。这就是startup_32。这个入口在arch/x86/boot/compressed/header_32.S里面。至此，请记住，bootloader对于kernel来说意义只有boot_params结构了，其他的一切的一切已经都是浮云了。而boot_params当前地址在esi中。

startup_32缺省在内存的绝对地址0x10000。让我们慢慢解析startup_32。

 #include
 #include
 #include
 #include
 #include
 #include
 __HEAD
 ENTRY(startup_32)
 cld
 /*
 * Test KEEP_SEGMENTS flag to see if the bootloader is asking
 * us to not reload segments. BP_loadflags(%esi)即指向boot_params.loadflags.这一位应该是在设置了code32_start Hook的时候使用，因为在protected_mode_jump的最后已经将所有的段都置为_BOOT_DS了。如果bootloader hook了code32_start，返回kernel的时候显然kernel需要去恢复所有的段。
 */
 testb $(1<<6), BP_loadflags(%esi)
 jnz 1f
 cli
 movl $__BOOT_DS, %eax
 movl %eax, %ds
 movl %eax, %es
 movl %eax, %fs
 movl %eax, %gs
 movl %eax, %ss
 1:
 /*
 * Calculate the delta between where we were compiled to run
 * at and where we were actually loaded at. This can only be done
 * with a short local call on x86. Nothing else will tell us what
 * address we are running at. The reserved chunk of the real-mode
 * data at 0x1e4 (defined as a scratch field) are used as the stack
 * for this calculation. Only 4 bytes are needed.
 */
 //仔细阅读了以上的这段英文，不能不说代码构思的巧妙。由于不知道代码是否被加载到0x100000的地址，通过以下的代码就能计算出实际加载的地址和预期地址的差异，也就是说是实际的startup_32的位置。
 leal (BP_scratch+4)(%esi), %esp //boot_params.scratch的地址设置成为堆栈顶。
 call 1f //boot_params.scratch里面就是1：的实际地址
 1: popl %ebp //ebp就是1:的实际地址
 subl $1b, %ebp //ebp-1:就是实际与预期的差异， 也就是说是实际的startup_32的位置。
 /*
 * %ebp contains the address we are loaded at by the boot loader and %ebx
 * contains the address where we should move the kernel image temporarily
 * for safe in-place decompression.
 */
 #ifdef CONFIG_RELOCATABLE
 movl %ebp, %ebx

 //kernel_alignment里面是kernel地址对齐所需要移动的位移量,这是有bootloader填入的，因为bootloader可能将startup_32装载在非对齐的地址。那么就需要增加移动的位移量来保证对齐而达到更好的性能。下面的代码就是要调整地址的位移而保证对齐。
 movl BP_kernel_alignment(%esi), %eax
 decl %eax
 addl %eax, %ebx
 notl %eax
 andl %eax, %ebx
 #else
 movl $LOAD_PHYSICAL_ADDR, %ebx //LOAD_PHYSICAL_ADDR在 arch/x86/include/asm/boot.h里.实际上应该是0x100000
 #endif
 /* Target address to relocate to for decompression */
 addl $z_extract_offset, %ebx //z_extract_offset由MKpiggy.c 在编译时产生的piggy.S里面定义。在我编译kernel时，z_extract_offset是0x4a0000，现在ebx的值在不考虑reloc的情况下是0x5a0000
 /* Set up the stack */
 leal boot_stack_end(%ebx), %esp //在0x5a0000+boot_stack_end的位置建立栈。
 /* Zero EFLAGS */
 pushl $0
 popfl
 /*
 * Copy the compressed kernel to the end of our buffer
 * where decompression in place becomes safe.
 */
 pushl %esi
 leal (_bss-4)(%ebp), %esi //esi指向源，即ebp+_bss-4的地址,是当前bootloader加载32位kernel的地址空间
 leal (_bss-4)(%ebx), %edi //edi指向目的地址，即ebx+_bss-4的地址，如果kernel不要reloc，就是0x5a0000+_bss-4
 movl $(_bss - startup_32), %ecx //从startup_32d到_bss有多少个字节？
 shrl $2, %ecx //实际我们移动每次4个字节，所以ecx需要除4.
 std
 rep movsl //走咯，我们把自己移动上去
 cld
 popl %esi
 /*
 * Jump to the relocated address.
 */
 leal relocated(%ebx), %eax
 jmp *%eax //跳转到relocated上去即ebx+relocated,即0x5a0000+relocated.
 ENDPROC(startup_32)
 .text
 relocated:
 /*
 * Clear BSS (stack is currently empty)
 */
 xorl %eax, %eax
 leal _bss(%ebx), %edi
 leal _ebss(%ebx), %ecx
 subl %edi, %ecx
 shrl $2, %ecx
 rep stosl
 /*
 * Adjust our own GOT GOT是什么？难道是Global Object Table?为什么GOT里面的每一个项都加上了ebx(0x5a0000)?难道里面是一堆指针需要调整所以加上ebx?
 */
 leal _got(%ebx), %edx
 leal _egot(%ebx), %ecx
 1:
 cmpl %ecx, %edx
 jae 2f
 addl %ebx, (%edx)
 addl $4, %edx
 jmp 1b
 2:
 /*
 * Do the decompression, and jump to the new kernel..
 */
 leal z_extract_offset_negative(%ebx), %ebp //ebp=ebx-0x4a0000=0x100000
 /* push arguments for decompress_kernel: */
 pushl %ebp /* output address */ //将Kernel解压缩到0x100000
 pushl $z_input_len /* input_len */ //压缩过的kernel大小
 leal input_data(%ebx), %eax //压缩kernel开始地址
 pushl %eax /* input_data */
 leal boot_heap(%ebx), %eax //工作的堆
 pushl %eax /* heap area */
 pushl %esi /* real mode pointer */ //esi是boot_params
 call decompress_kernel //我不准备去看怎么解压，只要知道它解压了好了
 addl $20, %esp //看来不需要恢复寄存器
 #if CONFIG_RELOCATABLE
 /*
 * Find the address of the relocations.
 */
 leal z_output_len(%ebp), %edi
 /*
 * Calculate the delta between where vmlinux was compiled to run
 * and where it was actually loaded.
 */
 movl %ebp, %ebx
 subl $LOAD_PHYSICAL_ADDR, %ebx
 jz 2f /* Nothing to be done if loaded at compiled addr. */ //如果ebx=0x100000，则不许要reloc
 /*
 * Process relocations. //这段没懂，但应该不影响理解
 */
 1: subl $4, %edi
 movl (%edi), %ecx
 testl %ecx, %ecx
 jz 2f
 addl %ebx, -__PAGE_OFFSET(%ebx, %ecx)
 jmp 1b
 2:
 #endif
 /*
 * Jump to the decompressed kernel.
 */
 xorl %ebx, %ebx
 jmp *%ebp //重新跳到0x100000开始。
 /*
 * Stack and heap for uncompression
 */
 .bss
 .balign 4
 boot_heap:
 .fill BOOT_HEAP_SIZE, 1, 0
 boot_stack:
 .fill BOOT_STACK_SIZE, 1, 0
 boot_stack_end:

把以上的代码总结一下其实很简单，startup_32将压缩过的kernel和本身移动到0x100000(或bootloader装载startup_32的地址)+0x4a0000的位置，然后解压缩kernel回到0x100000(或bootloader装载startup_32的地址)，然后将控制权交回到0x100000((或bootloader装载startup_32的地址)).

启动代码终于结束了，明天就要进入真正的kernel了。写得有些乱，就是对代码进行注释，暂时找不到更好的方法来对代码进行解释，可能使大家看起来有点累。希望对大家有帮助。